The evacuation of the combustion gases from a cylinder in an internal combustion engine during combustion operation may be based on two different mechanisms. When an exhaust valve opens near bottom dead center of the cylinder at the beginning of a combustion cycle, the combustion gases flow at high velocity through an exhaust port into the exhaust system. This is caused by the high pressure level prevailing in the cylinder toward the end of combustion and the associated high pressure difference between the combustion chamber and the exhaust section. This pressure-driven flow process is accompanied by a high pressure peak, which may be referred to as a preliminary exhaust surge and propagates along the exhaust line at the speed of sound, with the pressure degrading (i.e. decreasing) to a greater or lesser extent due to friction with increasing distance traveled and in a manner dependent on the routing of the exhaust lines.
As the combustion cycle progresses, the pressures in the cylinder and in the exhaust line balance each other out to a large extent, and the combustion gases are therefore expelled as a result of the reciprocating motion of the piston.
The dynamic wave processes or pressure fluctuations in the exhaust system in a multi-cylinder internal combustion engine having staggered combustion ignition operation may interfere with one another during the combustion process. This may be referred to as cylinder cross-talk. The interference between the cylinders may result in a reduced torque and power output.
Specifically, the pressure fluctuations in gaseous media propagated as waves passing through the exhaust lines may be reflected at the open or closed ends of the lines. The exhaust flow or local exhaust gas pressure in the exhaust system is then the product of the position of the advancing wave and the reflected wave. In some exhaust systems, the pressure waves emanating from a cylinder pass may pass through the exhaust line corresponding to the cylinder as well as along the exhaust lines of the other cylinders, possibly as far as the exhaust port provided at an end of the respective line.
Exhaust gas which has already been expelled or discharged into an exhaust line during the exhaust and refill process can thus reenter the cylinder, which may be caused by the pressure wave emanating from another cylinder. This interference may reduce the flowrate of exhaust gases from the cylinders into the exhaust lines, thereby degrading combustion operation. As a result, combustion efficiency and/or engine power output may be reduced.
It has been found that combustion operation is degraded, for example, if there is an excess pressure prevailing at the exhaust port of a cylinder or if the pressure wave of another cylinder is propagating in the direction of the exhaust port along the exhaust line toward the end of the exhaust and intake process, counteracting the evacuation of the combustion gases from this cylinder.
Cylinder cross-talk may be particularly prevalent at low engine speeds during valve overlap, when an intake valve is open and an exhaust valve is not yet closed. As a result, the efficiency and power output of the engine may be decreased. Therefore, the size of the engine may be increased to counteract the decreased power output, thereby increasing the size and cost of the engine while increasing the engine's fuel consumption.
Cross-talk may also become particularly problematic in engines having integrated exhaust manifolds. Specifically, in an integrated exhaust manifold all of the exhaust lines may be merged into a single conduit in the cylinder head, reducing the length travelled by exhaust gases through separate exhaust lines. As a result, the problem of mutual interference between the cylinders during the combustion process may be exacerbated.
Attempts have been made to reduce interference in engines having integrated exhaust manifolds by increasing the width of the cylinder head. However, widening the cylinder head may have adverse affects on the vehicle's impact absorbing characteristics. For example, free space in the engine compartment for deformation of the vehicle may be needed to achieve desired impact absorbing characteristics. However, widening the cylinder head reduces the space in the engine compartment.
Other, attempts have been made to reduce cross-talk by increasing the length of individual exhaust lines fluidly separated from other each-other in an engine having an exhaust manifold positioned outside of the cylinder head. However, this may increase the size of the engine as well as negatively affect turbocharger operation and catalyst operation.
As such in one approach, an engine having a cylinder head including an exhaust manifold at least partially integrated therein, the exhaust manifold including an inner separating wall fluidly dividing two merged exhaust lines, each merged exhaust line in fluidic communication with a different pair of adjacent cylinders, and an outer separating wall fluidly dividing a first exhaust line in direct fluidic communication with a first cylinder and a second exhaust line in direct communication with a second cylinder, a lateral width of the inner separating wall greater than the outer separating wall, the lateral axis perpendicular to a longitudinal axis traversing centerlines of each cylinder. It has been unexpectedly found that cross-talk between the cylinders in a cylinder head having these geometric characteristics is reduced while at least partially integrating the exhaust manifold into the cylinder head. As a result, a desired amount of engine compactness may be achieved while at the same time reducing interference between the cylinders caused by pressure fluctuations.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.
The invention is described in greater detail below with reference to three embodiments in accordance with FIGS. 1 to 4.